1. Field of the Invention
The present invention relates to an antenna device, and more particularly, is suitably applied to a portable telephone which is reduced in size.
2. Description of the Related Art
In small portable radio apparatus such as portable telephones, terminal units for personal handyphone system (PHS), or the like, a reduction in size, thickness and weight has been promoted with recent rapid development of such apparatuses. Correspondingly, antennas associated therewith are also required to have a reduced size, thickness and weight as well as higher performance.
An example of an antenna equipped in such a small portable radio apparatus is a micro-strip antenna (hereinafter referred to as the "MS antenna"). Further, commonly known as antennas, which are further reduced in size than the MS antennas, are a single-side short-circuited MS antenna having a short-circuit surface for short-circuiting a zero-potential surface at the center of a radiation conductor to a ground conductor, a laminar inverted-F antenna having a further reduced width of its short-circuit surface, and so on.
For example, as illustrated in FIGS. 1A and 1B, a conventional MS antenna 1 comprises a ground conductor 2 disposed on one side of a dielectric substrate 3 having a height h, and a rectangular radiation conductor 4 (length a.times.width b) formed on the other side of the substrate 3 using an etching technique or the like.
This MS antenna 1 is provided with a power supply point 5 at a predetermined position on the radiation conductor 4 so that the input impedance thereof is equal to the characteristic impedance of a power supply system. The MS antenna 1 operates as an antenna with power supplied thereto through the power supply point 5.
As illustrated in FIGS. 2A and 2B, a single-side short-circuited MS antenna 6 comprises a short-circuit conductor 10 having a width Ws1 identical to the width b of a radiation conductor 8 and a height h, disposed between the radiation conductor 8 and a ground conductor 7, so as to short-circuit a zero-potential surface of the radiation conductor 8 to the ground conductor 7. The zero-potential surface, at which an electric field is at "0," is at a position corresponding to one half a/2 of the length a of the radiation conductor 4 in the normal MS antenna 1.
With this structure, the single-side short-circuited MS antenna 6 only requires the radiation conductor 8 having a length dimension approximately one half of the length dimension of the radiation conductor 4 of the MS antenna 1 and still operates as an antenna at the same resonant frequency as the MS antenna 1.
Further, as illustrated in FIGS. 3A and 3B, a laminar inverted-F antenna 10 is composed of a rectangular radiation conductor 12 (length c.times.width d) and a ground conductor 11 which are short-circuited by a laminar inverted-F short-circuit conductor 14 having a width Ws2 smaller than the width Ws1 of the short-circuit conductor 10 of the single-side short-circuited MS antenna 6.
The laminar inverted-F antenna 10 can reduce the resonant frequency fr by virtue of the laminar inverted-F short-circuit conductor 14 having the width Ws2 chosen to be smaller than the width Ws1 of the short-circuit conductor 8 of the single-side short-circuited MS antenna 6, and can further reduce the resonant frequency fr by virtue of the power supply point 5 defined at a position offset from the center line of the radiation conductor 12 by an offset amount Wx2, as compared with the power supply point 5 defined at the center of the radiation conductor 12.
As mentioned above, since the laminar inverted-F antenna 10 is designed to reduce the resonant frequency fr more than the MS antenna 1, it can be configured using the radiation conductor 12 (length c.times.width d) smaller than the radiation conductor 4 (length a.times.width b), when it is operated at the same frequency as the MS antenna 1.
The single-side short-circuited MS antenna 6 and the laminar inverted-F antenna 10, configured as described above, are required to be further reduced in size in response to the demand for increasingly smaller portable telephones in recent years.